Flip chip power switch with under bump metallization stack

ABSTRACT

A semiconductor package includes a semiconductor substrate a semiconductor substrate having source and drain regions formed therein, an intermediate routing structure to provide electrical interconnects to the source and drain regions, a dielectric layer formed over the intermediate routing structure, and an under-bump-metallization (UBM) stack. The intermediate routing structure includes an outermost conductive layer, and the dielectric layer has an opening positioned over a portion of the intermediate layer routing structure. The UBM stack includes a conductive base layer formed over the dielectric layer and electrically connected to the outermost conductive layer through the opening, and a thick conductive layer formed on the base layer. A conductive bump is positioned on the UBM stack and laterally spaced from the opening.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/017,513, filed on Dec. 28, 2007, the entire disclosure of whichis incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to integrated circuits, and inone aspect to flip chip integrated circuits.

BACKGROUND

Voltage regulators, such as DC to DC converters, are used to providestable voltage sources for electronic systems. Switching voltageregulators are known to be an efficient type of DC to DC converter. Suchvoltage regulators typically include a power switch to generate arectangular-wave voltage that is then filtered to provide the output DCvoltage.

Conventionally, a power switch was fabricated on an integrated circuitchip with wire bond packaging. More recently, the power switch has beenfabricated as an integrated circuit chip with flip-chip packaging.

Conductive pads are formed on at least one surface of the chip toprovide electrical couplings to the integrated circuit components, e.g.,transistors, internal to the chip. Traditionally, the electricalcoupling between the transistors and the conductive pads is accomplishedthrough the use of multiple conductive routing layers that are formedover the semiconducting substrate of the integrated circuit chip butbelow an insulative layer that supports the conductive pads andseparates the pads from the routing layers (an aperture in theinsulative layer can provide electrical coupling of the contact pad tothe uppermost conductive routing layer). For example, an integratedcircuit chip may have metal lines and vias that electrically couple thesource and drain regions of the transistors to the conducting pads ofthe integrated circuit chip.

SUMMARY

In one aspect, a semiconductor package includes a semiconductorsubstrate a semiconductor substrate having source and drain regionsformed therein, an intermediate routing structure to provide electricalinterconnects to the source and drain regions, a dielectric layer formedover the intermediate routing structure, and an under-bump-metallization(UBM) stack. The intermediate routing structure includes an outermostconductive layer, and the dielectric layer has an opening positionedover a portion of the intermediate layer routing structure. The UBMstack includes a conductive base layer formed over the dielectric layerand electrically connected to the outermost conductive layer through theopening, and a thick conductive layer formed on the base layer. Aconductive bump is positioned on the UBM stack and laterally spaced fromthe opening.

Implementations may include one or more of the following features. Theconductive base layer may extend into the opening to contact theoutermost conductive layer. The conductive base layer may be formed onthe dielectric layer. A contact layer may be formed on the dielectriclayer, and the contact layer may extend into the opening to contact theoutermost conductive layer. The conductive base layer may be formed onthe contact layer. The dielectric layer may have a plurality of openingspositioned over the portion of the intermediate layer routing structure,the conductive base layer may be electrically connected to the outermostconductive layer through the openings, and the conductive bump may belaterally spaced from the openings. The conductive base layer mayinclude at least one material selected from titanium or anickel-vanadium alloy. The conductive base layer may be titanium and thethick conductive layer may be copper. A seed layer may be formed betweenthe conductive base layer and the thick conductive layer. The seed layermay be the same composition as the thick conductive layer, e.g., theseed layer and the thick conductive layer may be copper. The thickconductive layer may have a thickness greater than 6 μm, e.g., athickness greater than 9 μm, e.g., a thickness of about 9 to 14 μm. Athickness of the thick conductive layer may be sufficiently large so asto allow electrical current to propagate laterally along the surface ofthe thick conductive layer and through the electrical interconnects withlower resistance than conductive layers in the intermediate routingstructure. The source and drain regions may be coupled to the UBM stackthrough short lateral interconnects in the intermediate routingstructure. The dielectric layer may be a passivation layer that includessilicon-oxide or silicon-nitride materials. The substrate may include adistributed transistor and the package comprises a power switch. Thesource and drain regions may be part of a distributed transistor.

In another aspect, a semiconductor package includes a semiconductorsubstrate having source and drain regions formed therein, anintermediate routing structure to provide electrical interconnects tothe source and drain regions, a dielectric layer formed over theintermediate routing structure, and an under-bump-metallization (UBM)stack. The intermediate routing structure includes an outermostconductive layer, and the dielectric layer has an opening positionedover a portion of the intermediate layer routing structure. The UBMstack includes a titanium layer formed over the dielectric layer andextending into the opening to contact the outermost conductive layerelectrically, a seed layer formed on the titanium layer, and a copperlayer formed on the seed layer. A conductive bump is positioned on theUBM stack and laterally spaced from the opening.

Implementations may include one or more of the following features. Thesubstrate may include a distributed transistor and the package mayprovide a power switch.

In another aspect, a semiconductor package includes a semiconductorsubstrate having source and drain regions formed therein, anintermediate routing structure to provide electrical interconnects tothe source and drain regions, a dielectric layer formed over theintermediate routing structure, and an under-bump-metallization (UBM)stack. The intermediate routing structure includes an outermostconductive layer, and the dielectric layer has a plurality of openingspositioned over a portion of the intermediate layer routing structure.The under-bump-metallization (UBM) stack includes a conductive baselayer formed over the dielectric layer and electrically connected to theoutermost conductive layer through the openings, and a conductive layerformed on the base layer. A conductive bump is positioned on the UBMstack and laterally spaced from the openings.

Implementations can realize one or more of the following advantages. Apower switch can be constructed with lower resistance and reduced powerloss, and thus can handle increased current and/or have a reducedlikelihood of failure. The power switch can be constructed withoutsignificant increase in processing complexity.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is an exemplary semiconductor wafer.

FIG. 2 is a cross section view of an exemplary integrated circuit chipwith a bond pad and a conductive bump.

FIG. 3 is a schematic side view of an exemplary flip chip packagemounted on a printed circuit board.

FIG. 4 is a partial cross section view of an integrated circuit chip.

FIG. 5 is a schematic top view of an exemplary under-bump metallizationlayer and conductive bump.

FIG. 6 is a block diagram of an exemplary switching regulator.

FIG. 7 is a flow chart for fabricating an integrated circuit chip shownin FIG. 4.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Flip-chip packaging is an advanced packaging design for semiconductortechnology that allows an overall size of the packaged chip to be madevery compact. Instead of wire bonding, flip-chip technology formsconductive bumps on the bond pads on the planar surface of an integratedcircuit chip. The conductive bumps may include metals or alloys, e.g.,gold, silver, aluminum, copper, brass, or tin/lead alloys, or conductiveor conductor-filled epoxies. Where the chip uses ball grid array (BGA)packaging, the conductive bumps can be formed with balls of solder. Theintegrated circuit chip can be mounted directly on a printed circuitboard (PCB) that has contact electrodes corresponding to the locationsof the conductive bumps. Alternatively, the integrated circuit chip canbe mounted on a specialized lead frame having contact terminalscorresponding to the placement of the conductive bumps. In general, theintegrated circuit chip is placed face-down (or “flip-chip” bonded) sothat the conductive bumps can be placed on the contact electrodes of thePCB, an interposer between the chip and the PCB, or the contactterminals of the lead frame.

The conductive bumps are typically bonded to the PCB or lead frame usinga reflow process. During the reflow process, the conductive bumps areheated until they flow and bond with the corresponding contactelectrodes of the printed circuit board or contact terminals of the leadframe. Upon cooling, the conductive bump forms a mechanical terminal andserves as an electrical connection between the integrated circuit chipand the printed circuit board or lead frame.

Since no bonding wires are required, which would otherwise occupy alarge layout space, the overall size of the flip-chip package is verycompact as compared to conventional types of semiconductor devicepackages. In addition, a flip-chip package provides a short electricalpath from external devices to the internal components of the substrate,minimizing series parasitic impedances, which is beneficial forapplication optimization and high volume automated production.

FIG. 1 shows an exemplary semiconductor wafer prior to dicing. As shown,the wafer 100 includes an array of integrated circuit chips (or dice)102, and each chip 102 includes a plurality of bond pads 108 andconductive bumps 104 to provide electrical connections between theinternal semiconductor device components, e.g., transistors, andexternal components, e.g., other components on the PCB. Each integratedcircuit chip is separated from adjacent chips by vertical and horizontalscribe lines 106.

Typically, after the semiconductor wafer 100 is fabricated and contacts(e.g., bond pads 108) are formed thereon, the wafer 100 is diced usingconventional dicing machines, e.g., sawing or cutting the wafer, orother techniques, e.g., laser or etching, and singulated into individualchips along the scribe lines 106. Thus, each chip has its ownsemiconductor body, bond pads and conductive bumps. However, theconductive bumps can be formed on a chip after singulation.

The semiconductor wafer 100 can be a single crystal silicon body, or beformed of other materials such as gallium arsenide or silicon germanium.The bond pads 108 and conductive bumps 104 can be formed as an array ofcontacts, or be placed into adjacent rows and columns to form a grid orarray. Although the bond pads 108 are shown as square, they can haveother shapes, e.g., circular, rectangular, or other polygons. Inaddition, although the bond pads 108 are shown with uniform spacing inboth lateral directions across the chip, other configurations andpatterns are possible. For example, the pitch can be different in thedifferent lateral directions, the spacing can be non-uniform, or therecan be regions without bond pads. In one implementation, conductivebumps can be arranged, e.g., in a single or double row, around theperiphery of an integrated circuit chip.

Each integrated circuit chip 102 can include one or more switchingcircuits each of which includes one or more switches. Each switch can bea distributed transistor, e.g., an array of parallel transistors thatincludes multiple doped drain and source regions. The doped regions canbe arranged to form parallel stripes, a checkerboard pattern, or otherpatterns. The individual doped drain regions of a single distributedtransistor are connected by one or more conductive layers, e.g., metallayers, to form a common drain. Similarly, the individual doped sourceregions of a single distributed transistor are connected by one or moreconductive layers, e.g., metal layers, to form a common source.Overlying the interstitial region between the source and drain regionscan be one or more gate electrodes that function to control theswitching state of the distributed transistor. Further descriptionregarding the structure of the switch, such as the configuration of thedistributed transistor, can be found in, for example, U.S. Pat. No.6,713,823 the disclosure of which is incorporated herein by reference inits entirety.

FIG. 2 shows a cross section view of the integrated circuit chip 102with a bond pad 108 and conductive bump 104 (a single bond pad with asingle conductive bump is illustrated for simplicity). A singleconductive bump 104 can be formed on each bond pad 108, or multipleconductive bumps can be formed on each bond pad.

Referring to FIG. 2, the integrated circuit chip 102 includes asemiconductor substrate 210 in which the doped source and drain regionsare formed. The integrated circuit chip 102 also includes a routingstructure 112 between the bond pad 108 and the semiconductor substrate210. The routing structure 112 includes conductive layers to provideelectrical routing between the source and drain regions in thesemiconductor layer 210 and the conductive bond pad 108, and insulativelayers to electrically insulate the conductive layers from each otherand insulate traces within a layer from each other. The routingstructure 112 can include an outermost conductive layer 214, and anelectrically insulative passivation layer 410 can be formed over theoutermost conductive layer 214. An aperture 422 in the insulative layer410 can permit electrical connection of the bond pad 108 to theoutermost conductive layer 214.

While the conductive bump 104 is shown as a spherical ball, theconductive bump 104 also may be formed in other shapes such as, forexample, stud bumps or columnar members.

The contact pad 108 facilitates the coupling between the integratedcircuit chip 102 and devices and components electrically connected tothe conductive bump 104. The pad can be a square, circular, rectangular,elliptical, hexagonal, octagonal, or some other shape. In someimplementations, the contact pad 108 can include a multi-layeredmulti-stack structure. FIG. 4 shows such a structure, as will bediscussed in greater detail below.

In some implementations, the integrated circuit chip 102 can include, inaddition to a power switch, other functionality, such as one or moreapplication specific devices, memory devices or control devices.

FIG. 3 is a schematic side view of an exemplary flip chip package 316which includes an integrated circuit chip that can serve as a powerswitch, with the flip chip package mounted on a PCB 302. The mountedflip-chip package 316 includes the integrated circuit chip 102 and alead frame 304 and is mounted on the PCB 302. Other components of thevoltage regulator, such as, without limitation, output filters,interconnects and feedback circuits, can also be mounted on the PCB 302.

The lead frame 304 includes conductive leads 308. The integrated circuitchip 102, with conductive bumps 104 on contact pads 108 as shown in FIG.2, can be attached and electrically connected to the leads 308 of thelead frame 304. Optionally, an encapsulation material (not shown) can beapplied over the integrated circuit chip 102 to protect the chip againstexternal damage. The lead frame 304 can be configured to distributesignals from the integrated circuit chip 102 to appropriate terminals312 on the PCB 302.

The integrated circuit chip 102 can be connected to the top of theconductive leads 308 by conductive bumps 104, which may be gold, silver,aluminum, copper, brass, an alloy or eutectic of lead and tin, oranother metal or metal alloy, or conductive or conductor-filled epoxies.The bottom of the lead frame 304 can be connected to the PCB 302 byadditional conductive bumps, solder paste, or any other surface-mountassembly technique.

The conductive bumps 104 can be formed using known techniques such asvapor deposition of solder material or by ball bumping with wire bondingequipment. While the conductive bumps 104 are described in terms ofbumps, such connotations should not be construed as imposing a specificor particular shape, and that the conductive bumps 104 can be configuredas balls, columns, pillars, or other desired geometrical configurations.

FIG. 4 is a partial cross section view of an integrated circuit chip.Referring to FIG. 4, various parts and layers of the integrated circuitchip 102 are illustrated. One of ordinary skill in the art wouldunderstand that this figure depicts the integrated circuit chip prior tothe attachment (e.g., to the lead frame or PCB), and some processes,such as conductive bump placement, usually proceed before such anattachment. Additionally, the relative thickness of layers and bumpsshown is not drawn to scale, and is merely representative. It should beunderstood that the values and relationships of thicknesses of thesecomponents should not be based on those depicted in the figures.

As shown in FIG. 4, the integrated circuit chip 102 includes thesubstrate 210, an intermediate routing structure 112 including theoutermost conductive layer 214 and a innermost conductive layer 404, acontact layer 406, and a passivation layer 410, such as a dielectriclayer.

As discussed above, the substrate 210 includes a substrate, e.g., asemiconductive material, e.g., silicon or gallium arsenide, in which anumber of doped regions (e.g., source regions and drain regions) of adistributed transistor may be formed. For example, the doped regions canbe p-doped regions in an n-type semiconductor substrate layer or well ifthe distributed transistor is a PMOS transistor. As another example, thedoped regions can be n-doped regions in an p-type semiconductor layer orwell if the distributed transistor is an NMOS transistor. If the switchis to be an LDMOS transistor, then the drain regions can be n-dopedportions in a n-type substrate or well, and the source region caninclude both highly p-doped and highly n-doped portions in alightly-p-doped p-body in the n-type substrate or well.

A gate in the form of gate lines can be formed on the substrate 210.Assuming the aforementioned doped regions are arranged in a checkerboardpattern, the gate lines can surround each doped region to form arectangular array of doped regions each separated by the gate lines.Where the doped regions are in alternating rows, the gate lines canextend in parallel between the rows. The gate lines can be formed of aconductive material, e.g., polysilicon, that is separated from thesemiconductor body by an insulating layer, e.g., a gate oxide layer,such as a silicon dioxide.

Although two conductive layers of the intermediate routing structure 112are shown, the intermediate routing structure 112 can be a single (e.g.,a single conductive layer) or a multi-layered stack (e.g., two or moreconductive layers) structure. For example, the intermediate routingstructure 112 can include first, second and third conductive layers(with the third conductive layer being the outermost conductive layer214), and one or more insulating layers therebetween. The conductivelayers and the insulating layers may be placed in an alternatingfashion.

The first conductive layer can be formed directly on the substrate 210,and the second and third conductive layers can be deposited over thefirst conductive layers. The conductive layers can be formed of a metal,such as aluminum or copper, and the insulating layers can be formed ofan oxide, such as silicon oxide. In general, the outermost conductivelayer 214 allows current to flow through the underlying conductivelayers through a short path of vertical interconnects that help toreduce current flow density and resistance of interconnects, therebyreducing the power loss of the integrated circuit chip 102.

After the intermediate routing structure 112 is formed on the substrate210, the outermost conductive layer 214 can be passivated andelectrically insulated by depositing a passivation layer 410 over thesurface of the outermost conductive layer 214. The passivation layer 410can serve to separate conductive interconnect lines of the outermostconductive layer 214 from each other, and provide protection (e.g.,against physical damage, moisture, external contaminants) for theintegrated circuit chip 102.

The passivation layer 410 can be formed over an exposed surface of theoutermost conductive layer 214. The passivation layer 410 may be formedof, for example, silicon-based materials such as silicon oxides (e.g.,SiO₂) or silicon nitrides (e.g., SiN), or layers of other passivationmaterials which may be deposited by conventional sputtering or chemicalvapor deposition (CVD) processes.

One or more openings 422 can be etched or otherwise patterned within aportion of the passivation layer 410 that aligns with a partial surfaceof the outermost conductive layer 214 such that one or more sections ofthe top surface of the outermost conductive layer 214 are exposed. Theopenings 422 can be formed by depositing, for example, a photoresist andexposing the photoresist to define the location on the passivation film410 at which an opening is desired. The photoresist can be patterned byusing conventional photolithography methods, e.g., by spinning or bylaminating on a layer of photoresist, projecting light of a particularwavelength through a photomask with the desired pattern onto thephotoresist to expose the photoresist to the pattern, developing thephotoresist, removing the unexposed photoresist. The passivation layer410 can be etched through holes in the photoreist to establish theopening 422, and then the remaining photoresist can be removed.

The contact layer 406, which serves as an interface between anunder-bump-metallization (UBM) stack 408, also termed post-passivationmetallization, and the outermost conductive layer 214, is formed atopthe passivation layer 410 (in some implementations the contact layer 406is generally not considered part of the UBM, because the contact layercan define a bond pad suitable for wire bonding, but in someimplementations the contact layer 406 can be considered part of theUBM). The contact layer 406 can be provided on the passivation layer 410such that it penetrates into the opening 422 and contacts the topsurface of the outermost conductive layer 214. This establishes directcontact between the contact layer 406 and a segment of the outermostconductive layer 214.

The contact layer 406 can provide a plurality of contact pads on thesurface of the integrated circuit chip 102. The contact pads can bearranged in a regular array, e.g., a rectangular array. The contact padscan be of any shape suitable (e.g., square, circular, rectangular) forattachment of the solder bump, and can be formed of a conductivematerial such as a metal, e.g., aluminum. The size of the contact padsmay vary based on the design parameters of a particular designapplication, and therefore may vary between different applications. Insome implementations, the contact layer 406 has a thickness of about 3μm.

Once the contact layer is patterned, e.g., to define the contacts pads,an under-bump-metallization (UBM) stack 408 can be deposited over thepassivation layer 410 and the contact layer 406. The UBM stack 408 andthe layers thereof can collectively serve as a protective barrier,adhesion vehicle and wetting enhancer between the contact layer 406 andthe conductive bump 104. The UBM stack 408 can be formed on the contactlayer 406, and can extend in or over and around the opening 422. The UBMstack 408 can include one or more stack layers on which the conductivebump 104 is formed.

Optionally, the edges of the contact layer 406 can be covered with anadditional passivation layer prior to formation of the UBM stack 408. Inthis case, some of the UBM stack is formed mostly directly on thecontact layer 406, but some will extend onto the passivation layer.

A base layer 414 of the UBM stack 408 may be formed on the contact layer406, and a portion thereof can align with the opening 422 of thepassivation layer 410.

In the illustrative implementation, the UBM stack 408 is a three-layeredstructure which includes the base layer 414, a seed layer 416 and athick conductive outer layer 418. In one implementation, the base layer414 is formed from titanium. On the titanium layer a thin conductiveseed layer 416, e.g., of copper, is deposited, e.g., by sputtering. Thena thick conductive outer layer 418 is deposited, e.g., by plating, onthe seed layer. The thick conductive outer layer 418 can be a highlyconductive metal of the same composition as the seed layer, e.g.,copper. The thick conductive outer layer can be more than 6 micronsthick, e.g., more than 9 microns thick, e.g., about 12 to 14 micronsthick. The thick copper layer prevents electrical discontinuity fromdeveloping due to consumption of the copper layer during formation ofthe bump.

In another implementation, the UBM stack 408 is a two-layered (or more)structure. In this implementation, a conductive intermediate layer, suchas nickel-vanadium alloy forms the base layer. An outer conductivelayer, e.g., a copper layer, is then deposited on the intermediatelayer. The copper layer can be 0.8 micron or more, e.g., up to 14microns thick. The seed layer 416, which may include materials such ascopper, can be formed on the base layer 414, e.g., by sputtering, andthe thick conductive outer layer 418 can be formed, e.g., byelectroplating, atop the seed layer 414. The thickness of the conductiveouter layer 418 can be more than 6 microns thick, e.g., more than 9microns thick, e.g., about 12 microns thick.

It should be understood that the UBM stack 408 may include a greater orlesser number of layers than those shown, and that each layer caninclude alloys of various metals or materials other than those describedabove. For example, in some implementations, contact layer 406 and baselayer 414 can be a single layer, e.g., formed from titanium.

The UBM layers 414-418 can be formed using a suitable deposition processsuch as electroplating, sputtering or vapor deposition, and can beformed such that the conductive bump 104 can be properly placed thereonand adhered to the copper layer 418, resulting in a conductive bump thatis electrically coupled and connected to the contact layer 406. Thelateral cross-sectional area of the UBM stack 408 also can match theunderlying contact layer 406, or can otherwise be of a shape suitablefor attachment of the solder bump (e.g., square, circular, rectangular).

In some implementations, the thickness of the thick conductive layer 418is sufficiently large so as to allow current to propagate laterally withlower resistance than the inner conductive layers (e.g., conductivelayers other than outermost layer 214) in the intermediate routingstructure 112.

The UBM stack 408 can be formed over the contact layer 406 to allow forbetter bonding and wetting of the conductive bump 104 to the uppermostUBM layer (e.g., copper layer 418), and for protection of the contactlayer 406 and the outermost conductive layer 214 by the lowermost UBMlayer (e.g., the aluminum layer 214).

As noted above, the conductive bump 104 provides an electricalconnection to a PCB, an interposer between the chip and the PCB, or leadframe with flip-chip bonding. The conductive bump 104 may be formed by,for example, vapor deposition of solder material over the UBM stack 408.Alternatively, the conductive bump 104 may be formed by a solder-pastescreen printing method using a photoresist mask (not shown) to guide theplacement of the solder-paste. The conductive bump 104 may be formedafter depositing solder materials, for example, in layers or as ahomogeneous mixture, and removing the photoresist mask defining thelocation of the solder materials. A solder reflow process heats thesolder materials (e.g., to or somewhat above a melting point) until thematerials flow and bond with corresponding contact electrodes of a PCB,an interposer, or a lead frame. Upon cooling, the solder materialsharden and solidify in the form of a conductive bump which serves asboth mechanical and electrical connections between the carrier substrateand the semiconductor device 102. In certain implementations, increasingthe number of conductive bumps can lower the resistance of theconnection between the integrated circuit chip 102 and the carriersubstrate to which the integrated circuit chip 102 is connected.

FIG. 5 is a schematic top view of an UBM stack with a conductive bump.Referring to FIG. 5, the UBM stack 408, which overlies the outermostconductive layer 214, includes one or more first conductive areas 562 aand one or more second conductive areas 562 b, with the first areas 562a electrically isolated from the second areas 562 b. The firstconductive area 562 a and the second conductive area 562 b can simply bephysically separated structures on the top surface of the integratedcircuit chip (e.g., separated by an air gap), or an insulation layer canbe provided in the lateral space between the two areas 562 a and 562 b.

Multiple vias, e.g., vias 570 and 572, between the UBM stack 408 and theburied conductive layers inside the intermediate routing structure 112can be provided by apertures in the passivation layer 410 (e.g., opening422) that connect to conductive islands in the outermost conductivelayer 214, which are connected by further vias to the buried conductivelayers. Vias 570 and 572 can be connected to different buried layers inthe routing structure 112 (e.g., vias 570 can be connected a firstconductive layer and vias 572 can be connected to a second conductivelayer). At least some of the vias 570 and 572 are not directly under theconductive bump 104.

An edge of the first conductive area 562 a adjacent the secondconductive area 562 b has one or more lateral protrusions 550, e.g., asa plurality of rectangular protrusions that extended toward the secondconductive area 562 b. Similarly, an edge of the second conductive area562 b adjacent the first conductive area 562 a has one or more lateralprotrusions 552, e.g., a plurality of rectangular protrusions thatextended toward the first conductive area 562 a. The rectangularprotrusions of the two conductive areas can interlace in an alternatingpattern to form a region of interdigitated protrusions at the commonedge of the conductive areas 562 a and 562 b.

The first conductive area 562 a of the UBM stack 408 may substantiallyoverlap a first conductive region of the outermost conductive layer 214,and the second conductive region 564 b of the UBM stack 408 maysubstantially overlap a second conductive region of the outermostconductive layer 214.

The integrated circuit structure as described herein can couple thesource and drain regions of the distributed transistor to conductiveplanes of the UBM stack 408 with short lateral interconnects in theconductive layers inside the routing structure 112. Because theoutermost conductive layer 214 and the UBM stack 408 can be relativelythick, they can carry large amounts of current (in comparison to theother buried conductive layers inside the routing structure 112). Inparticular, the UBM stack 408 can carry current laterally to vias (e.g.,vias that are not directly under the conductive bump 104) with lowerresistance than the other buried conductive layers (e.g., innermostconductive layer 404) inside the routing structure 112. The planarcurrent flow in the outermost conductive layer 214 and the UBM stack 408and the short path of direct vertical interconnects reduce the currentflow density and resistance of interconnects. The design of the UBMstack 408 can therefore reduce power loss of the integrated circuit chip102 and improves the reliability of circuits and devices coupled to thechip 102. If conducting pads can be placed above the functional area ofthe integrated circuit chip 102, the area necessary for providingconducting pads on the integrated circuit chip 102 can be reduced andcheaper dice in a smaller package than a traditional integrated circuitstructure can be realized.

FIG. 6 is a block diagram of an exemplary switching regulator. Referringto FIG. 6, an implementation of a switching regulator 610 can be coupledto a DC input voltage source 612, such as a battery, by an inputterminal 620. The switching regulator 610 also can be coupled to a load614, such as an electronic device, by an output terminal 624.

The switching regulator 610 serves as a DC-to-DC converter between theinput terminal 620 and the output terminal 624. The switching regulator610 includes a switching circuit 616 that serves as a power switch foralternately coupling and decoupling the input terminal 620 to anintermediate terminal 622. The switching circuit 616 can include arectifier, such as a switch 632 or diode, coupling the intermediateterminal 622 to ground.

The switching circuit 616 and the output filter 626 may be configured ina buck converter topology with a first transistor 630 connected betweenthe input terminal 620 and the intermediate terminal 622 and a secondtransistor 632 connected between ground and the intermediate terminal622. The switching regulator 610 may also include an input capacitor 638connected between the input terminal 620 and ground.

The switching regulator also includes a controller assembly with a pulsemodulator 618 for controlling the operation of the switching circuit616. The pulse modulator 618 causes the switching circuit 616 togenerate an intermediate voltage having a rectangular waveform at theintermediate terminal 622. Although the pulse modulator 618 and theswitching circuit 616 are illustrated and described below as a pulsewidth modulator, various pulse frequency modulation schemes also arecontemplated.

The intermediate terminal 622 can be coupled to the output terminal 624by an output filter 626. The output filter 626 converts the rectangularwaveform of the intermediate voltage at the intermediate terminal 622into a substantially DC output voltage at the output terminal 624.Specifically, in a buck-converter topology, the output filter 626includes an inductor 634 connected between the intermediate terminal 622and the output terminal 624 and a capacitor 636 connected in parallelwith the load 614.

During a first conduction interval, the input voltage source 612supplies energy to the load 614 and the inductor 634 via the firsttransistor 630. On the other hand, during a second conduction interval,transistor 632 is closed and the inductor 634 supplies the energy. Theresulting output voltage V_(out) can be a substantially DC voltage.Although the switching circuit 616 and the output filter 626 areillustrated in a buck converter topology, other switching voltageregulator topologies, such as a boost converter topology, a buck-boostconverter topology or various transformer-coupled topologies, also areapplicable.

The output voltage can be regulated, or maintained at a substantiallyconstant level, by a feedback loop in the controller assembly thatincludes a feedback circuit 628. The feedback circuit 628 includescircuitry that measures the output voltage and/or the current passingthrough the output terminal. The measured voltage and current are usedto control the pulse modulator 618 so that the output voltage at theoutput terminal 624 remains substantially constant.

The conductive routings as described herein can be used in the switchingcircuit 616 to provide a flip-chip package that includes the switchingcircuit 616 and provides external couplings. In general, each switch(e.g., transistors 630 and 632) in the switching circuit 616 can befabricated as a distributed array of parallel transistors, and theconductive routing structures discussed above can carry current from thedoped source and drain regions of the switches to the conducting pads onthe surface of an integrated circuit chip that encompasses the switches.

For example, if transistor 632 is implemented as an n-channel device, itcan include rectangular n-doped source regions and drain regions laidout in a checkerboard pattern in a p-type well or substrate. Transistor630 may be constructed similarly. If transistor 630 is implemented as ap-channel device, it can include alternating rectangular p-doped sourceregions and drain regions in an n-type well or substrate. If transistor630 is implemented as an n-channel device, it can include alternatingrectangular n-doped source regions and drain regions in a p-type well orsubstrate. A grid-like gate may be implemented to separate each pair ofsource and drain regions. Electrical connection to the gate can beprovided at the peripheral edge of the chip.

FIG. 7 shows a process for fabricating the integrated circuit chip 102shown in FIG. 4. Referring to FIG. 7, the process begins with formingone or more transistors on a substrate (step 702).

An intermediate routing structure is formed on the substrate (step 704).The intermediate routing structure may be a multi-layered structure thatincludes conductive and insulating layers stacked in a configurationthat allows electrical current to travel to the substrate.

A dielectric layer is formed on the intermediate routing structure (step706). The dielectric layer can be a passivation film that includesinsulating properties. In some implementations, the dielectric layer mayinclude silicon-based materials such as silicon oxides or siliconnitrides, which may be deposited by conventional sputtering or chemicalvapor deposition processes. The passivation layer can be patterned tocreate one or more openings to expose a portion of the outermostconductive layer of the intermediate routing structure. For example, thepassivation layer can be pattered by applying a photoresist mask andthen etching exposed portions of the passivation layer. The passivationlayer may be patterned to fit a design profile for conductive bumpplacement. For example, the passivation may be patterned to create anopening having a circular, elliptical or oval shape.

Next, a contact layer is formed on the dielectric layer (708). Thecontact layer may be in a form of a contact pad that contains aluminum.The contact layer may be formed over the exposed portion of theintermediate layer and coupled to integrated circuit structures withinthe substrate.

Then the UBM stack is formed. The UBM stack may be a two or threelayered stack, and can function to provide barrier properties for theunderlying layers and pads. The UBM stack may be formed by usingconventional vapor deposition and electroplating techniques. In someimplementions, forming the UBM includes forming a metal base layer onthe contact layer (710), forming a seed layer on the first metal layer(712) and forming a second metal layer on the seed layer (714). In someimplementations, the first metal layer is titanium, the seed layer is afirst copper layer deposited by sputtering, and the second metal layeris a thick copper layer deposited by plating.

A conductive bump is formed on the second metal layer (716). Theconductive bump may include one or more layers of solder-wettablematerial or other conductive materials. The conductive bump may beformed using known techniques such as screen printing. The conductivebump may be in the form of a ball, column, pillar or other geometricalshape. The conductive bump may be provided by applying a solder maskover the second metal layer. The solder mask may be spun on or may be adry film laminate applied by a squeegee or a paste applied with astencil, or an ink material applied by screen printing. The conductivebump may be provided after patterning the solder mask.

The implementations as described above, and variations thereof, providewafer-level or chip-scale packaging for integrated circuit chips anddice. Although the above implementations have been depicted anddescribed with respect to the illustrated figures, various additions,deletions and modifications also are contemplated within its scope, andit should be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. For example,although only a single contact pad, UBM stack and conductive bump areshown for purposes of illustration, any integrated circuit chip andpackage can contain more than one of these components. As anotherexample, while a layer of aluminum, a layer of nickel-vanadium and acopper layer have been disclosed as exemplary UBM layers, other metalsand alloys also may be implemented for use in such layers. Accordingly,other implementations are within the scope of the following claims.

1. A semiconductor package, comprising: a semiconductor substrate havingsource and drain regions formed therein; an intermediate routingstructure to provide electrical interconnects to the source and drainregions, the intermediate routing structure including an outermostconductive layer; a passivation layer formed over the intermediaterouting structure, the passivation layer having a plurality of openingspositioned over a portion of the intermediate layer routing structure;an under-bump-metallization (UBM) stack including a conductive baselayer formed over the passivation layer and electrically connected tothe outermost conductive layer through the plurality of openings in thepassivation layer, and a conductive layer formed on the base layer, theconductive layer being thicker than the base layer; and a conductivebump positioned on and directly contacting the UBM stack and laterallyspaced from an opening of the plurality of openings in the passivationlayer such that at least one opening of the plurality of openings is notdirectly under a region of contact between the conductive bump and theUBM stack and at least another opening of the plurality of openings isdirectly under the region of contact between the conductive bump and theUBM stack.
 2. The semiconductor package of claim 1, wherein theconductive base layer extends into the opening to contact the outermostconductive layer.
 3. The semiconductor package of claim 2, wherein theconductive base layer is formed directly on the passivation layer. 4.The semiconductor package of claim 1, further comprising a contact layerformed on the passivation layer, the contact layer extending into theopening to contact the outermost conductive layer.
 5. The semiconductorpackage of claim 4, wherein the conductive base layer is formed on thecontact layer.
 6. The semiconductor package of claim 1, wherein theconductive base layer includes at least one material selected fromtitanium or a nickel-vanadium alloy.
 7. The semiconductor package ofclaim 6, wherein the conductive base layer consists of titanium and theconductive layer consists of copper.
 8. The semiconductor package ofclaim 1, further comprising a seed layer formed between the conductivebase layer and the conductive layer.
 9. The semiconductor package ofclaim 8, wherein the seed layer includes a same composition as theconductive layer.
 10. The semiconductor package of claim 8, wherein theseed layer and the conductive layer consist of copper.
 11. Thesemiconductor package of claim 1, wherein the conductive layer has athickness greater than 6 μm.
 12. The semiconductor package of claim 11,wherein the conductive layer has a thickness greater than 9 μm.
 13. Thesemiconductor package of claim 12, wherein the conductive layer has athickness of about 9 to 14 μm.
 14. The semiconductor package of claim 1,wherein a thickness of the conductive layer is sufficiently large so asto allow electrical current to propagate laterally along the surface ofthe conductive layer and through the electrical interconnects with lowerresistance than conductive layers in the intermediate routing structure.15. The semiconductor package of claim 1, wherein the source and drainregions are coupled to the UBM stack through short lateral interconnectsin the intermediate routing structure.
 16. The semiconductor package ofclaim 1, wherein the passivation layer includes silicon-oxide orsilicon-nitride materials.
 17. The semiconductor package of claim 1,wherein the substrate includes a distributed transistor and the packagecomprises a power switch.
 18. The semiconductor package of claim 1,wherein the source and drain regions are part of a distributedtransistor.
 19. The semiconductor package of claim 1, wherein theconductive base layer comprises a titanium layer, wherein the UBM stackfurther comprises a copper seed layer formed on the titanium layer, andwherein the conductive layer comprises a copper layer formed on the seedlayer, the copper layer having a thickness of about 9 to 14 microns. 20.A semiconductor package, comprising: a semiconductor substrate havingsource and drain regions formed therein; an intermediate routingstructure to provide electrical interconnects to the source and drainregions, the intermediate routing structure including an outermostconductive layer; a passivation layer formed over the intermediaterouting structure, the passivation layer having a plurality of openingspositioned over a portion of the intermediate layer routing structure;an under-bump-metallization (UBM) stack including a conductive baselayer formed over the passivation layer and electrically connected tothe outermost conductive layer through the openings in the passivationlayer, and a conductive layer formed on the base layer; and a conductivebump positioned on and directly contacting the UBM stack and laterallypositioned relative to the openings such that at least one opening ofthe plurality of openings is not directly under a region of contactbetween the conductive bump and the UBM stack and at least anotheropening of the plurality of openings is directly under a region ofcontact between the conductive bump and the UBM stack.
 21. Thesemiconductor package of claim 20, wherein each opening of multipleopenings from the plurality of openings is not directly under a regionof contact between the conductive bump and the UBM stack.
 22. Thesemiconductor package of claim 20, wherein the conductive bump issolder.
 23. The semiconductor package of claim 22, wherein theconductive bump is a solder ball.
 24. The semiconductor package of claim20, wherein the conductive bump is copper.
 25. The semiconductor packageof claim 24, wherein the conductive bump is a copper pillar.
 26. Thesemiconductor package of claim 20, wherein the conductive base layercomprises a titanium layer, wherein the UBM stack further comprises acopper seed layer formed on the titanium layer, and wherein theconductive layer comprises a copper layer formed on the seed layer, thecopper layer having a thickness of about 9 to 14 microns.